Devices are known for microwave-sterilization of medical instruments such as dental instruments--for example, those taught in PCT WO 93/18798. Typically such a design uses a pouch sealed with the instruments inside, which is positioned with at least the instrument portion of the pouch between two microwave shield portions, the entire combination being locked between two portions, preferably hinged together, of a plastic cassette. In any such device, an important component is the noted microwave shield that is disposed around the metal instruments, preferably using two clam-shell portions that come into contact at mating "faces" when closed to provide a shielded chamber inside. As is well-known, the shield prevents the transmission of microwave radiation to the metal instruments, since the unrestricted transmission of such would cause arcing of the instruments and surface damage. Another example of such shielding is taught in U.S. Pat. No. 5,248,478, wherein two small hemispherical metal portions are used to shield contact lenses rather than medical instruments.
We have discovered that merely surrounding instruments with a "closed" metal shield having "choke" edges in facing contact is not sufficient to prevent all arcing within the chamber formed by the shield. There are several reasons why a choke created by nominally contacting faces of two shield portions is not enough, but in all cases it is primarily due to the fact that the contact between the shield facing portions cannot be sufficiently maintained during microwaving. Specifically we have discovered that a) the expansion pressure of the sterilizing steam forces the "contacting" faces of the 2 shield portions to gap apart, and b) the build-up of dirt, dust or other particles in the "contacting" faces prevents the uniform contact in the first place, upon repeated use.
Such gap formation, though nominally small, leads to several detrimental results: First, the large sizes needed for medical instruments makes this case different from a small shield of the type described in U.S. Pat. No. 5,248,478 discussed above, in that unequal electrical potentials are created in the two shield portions due to uneven microwave energy distributions in the oven. These in turn cannot be equilibrated between the shield portions due to the gap, and arcing across the chamber inside the shields can occur. Second, the gap "leaks" microwave energy into the chamber, sufficient to create the risk of arcing between metallic tools being sterilized. Third, the gap induces arcing between the "contacting" faces of the shield portions, which due to the proximity of the sealed pouch inside the shielded chambers containing the tools, damages the pouch seal.
The difficulty then, prior to this invention, has been to construct a shield of significant size in two closable, contacting portions which, when closed, maintain sufficient contact during microwaving as to provide the effective electrical equilibration between shield portions that is necessary to neutralize uneven electrical potentials that tend to build up, eliminate "leaking" of microwave energy into the shielded chamber, and prevent arcing between the contacting faces, now gapped apart under pressure.